Today, mobile telephone systems usually are cellular systems, in which each cell in the system has at least one corresponding associated base station with at least one antenna for transmitting and receiving signals to/from e.g. user terminals of the system.
The base station antennas are designed such that the inclinational angle of the beam radiated from such an antenna generally is deflected downwardly with an angle relative to a horizontal plane in order to define a specific cell size. However, due to e.g. geographical topology and/or presence of buildings, the cell size in the system may vary, and so may the mounting height of the base station antennas. Therefore, the deflection angle, hereinafter referred to as downtilt angle, of the various antennas in the system must be set to different angles depending on the size of the particular cell in which the antenna is located, as well as the mounting location of the antenna.
The cell size, and thus also the downtilt angle, may also vary with varying kinds of cellular mobile telephone systems since different systems use different frequency ranges, and depending on the specific frequency range that is used, cell sizes have to be varied to provide a sufficient communication capacity.
The base station antennas are usually provided with a plurality of radiating elements arranged on a vertical row, and to vary the downtilt angle, a phase angle difference between the radiating elements is imposed on a common signal fed to the radiating elements, wherein the phase angle differences between any two elements is the same. This results in a composite beam from the plurality of radiating elements that will always have a wave front substantially in the form of a straight line. The inclination angle may further be adjustable, for example by means of phase shifters, by adjusting the phase angle difference between the radiating elements.
Today, adjustment of the phase shifters often requires that adjustment is carried out manually directly on or at the antenna, usually by maneuvering an operating element such as knob or a rod. Maneuvering the knob or rod may then actuate phase shifting means to relatively change the phase angle difference between signals fed to the radiating elements and thus the downtilt angle. There also exists, however, systems where the downtilt angle may be controlled from a remote location, e.g. by sending commands from a central operation and maintenance centre to control electronics associated with operating element actuating means, such that the control logic may translate e.g. a SET TILT=15° command to relative movement of the operating element actuator to perform a corresponding movement of the operating element, thus causing the phase shifting elements to effect a phase shift resulting in the desired down tilt angle.
One such system is previously known from the document EP EP1356539 (Kathrein Werke KG). EP1356539 discloses an antenna control apparatus as well as an associated antenna. The control apparatus has control electronics and an electric motor. The antenna control apparatus is arranged such that it can be retrofitted outside the protective cover of a base station antenna and engage an operating element, which is passed out of the interior of the antenna via an operating opening, or be introduced into the interior of the protective cover via this operating opening. Alternatively, the control apparatus may be fitted as a preferably complete unit underneath the protective cover of the antenna. The possibility of retrofitting a control apparatus is desirable since it makes it possible to modify existing antennas at existing base stations with only manual downtilt possibilities so as to enable remote downtilt control of those antennas.
One problem with existing remote tilt systems, however, is that the phase shifters that are used in remote tilt systems are rather complex and use mechanical solutions which require a substantial torque to manoeuvre the operating element.